1. Field of the Invention
The present invention relates to an image heating apparatus which heats an image formed on a recording medium such as paper.
2. Description of the Related Art
In the related art, an image forming apparatus such as a copying machine, a printer, a facsimile, or a multifunction machine of these, which uses an electrophotographic method, includes a fixing unit which is an image heating apparatus that heats and presses a toner image formed on a recording medium, and then fixes the image on the recording medium. In addition, as such a fixing unit, there is an apparatus having a configuration in which the glossiness of an image on a recording medium is adjusted by heating and pressing again the recording medium to which the image is fixed, in addition to fixation of the toner image on the recording medium.
As a configuration of such a fixing unit, a configuration in which a nip portion that pinches and conveys a recording medium is formed between a fixing roller serving as a rotatable member and an endless belt is known in the related art. For example, in JP-A-11-2979, a configuration in which a nip portion is formed between a fixing roller and an endless belt by arranging a pressure pad and a separation roller in the endless belt and separately biasing the pressure pad and the separation roller has been proposed.
In this configuration, the recording medium is separated from the fixing roller by a curvature of the separation roller.
In the case of the configuration disclosed in JP-A-11-2979 described above, the separation roller can move relatively with respect to the pressure pad in order to separately bias the pressure pad and the separation roller. For this reason, a predetermined gap is necessary to be provided between the separation roller and the pressure pad. In such a configuration, for example, in a case where a recording medium having low air permeability such as coated paper is used, an icicle-like portion with low gloss (for example, see FIG. 10, hereinafter referred to as gloss unevenness) can be generated on an image.
That is, in the case of the recording medium with low permeability, such as coated paper, an image disturbance occurs due to the expansion of the air in the inside of the nip portion or the generation of water vapor. More specifically, the air existing in a non-fixed toner layer stays in a gap between the pressure pad, which is a portion with low pressure of the nip portion, and the separation roller. In addition, the air allows a void to be partially generated in the interface between the recording medium and the fixing roller and disturbs the toner image, which is not completely fixed so that the above-described gloss unevenness is generated.